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Quando ha saputo la notizia, è rimasto di sasso.
When he heard the news, he was stunned.
To be completely shocked or stunned by something unexpected or surprising.
La sua reazione mi ha lasciato di sasso.
His reaction left me stunned.
Rimase di sasso quando vide il risultato del test.
He was shocked when he saw the test results.
The phrase comes from the idea of being so shocked that one feels as if turned to stone (sasso).
This idiom is commonly used in informal contexts to express strong surprise or shock.